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Heavy dust obscuration of z=7 galaxies in a cosmological hydrodynamic simulation

Hubble Space Telescope observations with Wide Field Camera 3/IR reveal that galaxies at z~7 have very blue ultraviolet (UV) colors, consistent with these systems being dominated by young stellar populations with moderate or little attenuation by dust. We investigate UV and optical properties of the high-z galaxies in the standard cold dark matter model using a high-resolution adaptive mesh refinement cosmological hydrodynamic simulation. For this purpose, we perform panchromatic three-dimensional dust radiative transfer calculations on 198 galaxies of stellar mass 5x10^8-3x10^{10} Msun with three parameters, the dust-to-metal ratio, the extinction curve, and the fraction of directly escaped light from stars (\fesc). Our stellar mass function is found to be in broad agreement with Gonzalez et al., independent of these parameters. We find that our heavily dust-attenuated galaxies (A_V~1.8) can also reasonably match modest UV-optical colors, blue UV slopes, as well as UV luminosity functions, provided that a significant fraction (~10%) of light directly escapes from them. The observed UV slope and scatter are better explained with an Small Magellanic Cloud-type extinction curve, whereas Milky Way-type dust predicts too blue UV colors due to the 2175A bump. We expect that upcoming observations by ALMA will be able to test this heavily obscured model.